With no intervention, this would rise over the 5 million a year mark by 2007, said the researchers.

If the necessary programmes were put in place, they said, this would eventually fall to approximately 1.5 million new infections a year.

Bleak future

By 2010, this would mean 28 million new infections, but under the worst case scenario, as many as 45 million people would fall prey to HIV.

The prospects for many of these would be bleak with little prospect of access to modern antiretroviral drugs.

David Serwadda, of Makerere University in Kampala, Uganda, and co-chair of the working group, said: "We failed to act decisively in the early stages of the epidemic in sub-Saharan Africa, and now we are paying the price.

"But we still have an opportunity to save the next generation in Africa from Aids, and to prevent runaway epidemics in India, Russia and China."

It would be possible, said the group, to prevent many deaths by "aggressively scaling up" existing, effective prevention programmes, such as condom promotion, mass media campaigns, and school programmes.

In addition, their report calls for billions more to be ploughed into vaccine research and research into anti-HIV microbicides.

Another key aim is to increase the involvement of political leaders in anti-HIV strategies.

Helene Gayle, director of the Bill and Melinda Gates Foundation's HIV/Aids programme, and co-chair of the group, said: "There is clearly a major gap in access to prevention worldwide.

"While we treat those infected, while we search for a vaccine, we have to embark on an unprecedented expansion of existing, proven HIV prevention programmes."

Sudden fall

There is evidence of the power of prevention from countries where HIV infection might have been expected to still be rising.

Another paper in this week's Lancet points to a fall in the HIV incidence rate in rural Uganda as evidence that prevention strategies can work.

Researchers found that the rate fell from 8.0 people per 1000 "person years" to 5.2 between 1990 and 1999.

However, Dr Justin Pankhurst from the London School of Hygiene and Tropical Medicine, said that the Ugandan success might not be replicated elsewhere.

He said: "Countries with HIV-1 prevalence rates of more than 30%, such as some in southern Africa, would be wrong to assume that by simply copying a few obvious Ugandan government interventions, they can expect to see a two-thirds reduction in their HIV-1 prevalence rate."